Regulatory Assets, Liabilities, And Balancing Accounts (Long-Term Regulatory Liabilities) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	$ 5,107	$ 4,733
Cost Of Removal Obligations [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	3,595	3,460
Recoveries In Excess Of AROs [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	649	611
Public Purpose Programs [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	613	499
Other Long-Term Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	$ 250	$ 163